Blood is made up of two fractions: blood cells and the liquid in which the blood cells are suspended. The liquid is known as blood plasma (“plasma”) and is a proteinaceous fluid which may include other smaller cells and dissolved proteins, such as serum albumins, globulins, and fibrinogen), glucose, clotting factors, electrolytes (Na+, Ca2+, Mg2+, HCO3−, Cl−, etc.), hormones, and carbon dioxide. The main cell types present in blood are the red blood cells (erythrocytes), white blood cells and platelets, although other cell types may be detected as well. Erythrocytes constitute about half of the volume of a blood sample.
The separation of red blood cells from the plasma in a blood sample is of great importance for the purpose of rapid diagnosis of plasma constituents in clinics, hospitals and in field conditions, because red blood cells can weaken the sensitivity of the assay. Also the whole blood can cause undesired chemical interference. For example hemoglobin that is released from red blood cells can affect the performance of certain clinical assays due to the iron heme group, which can act as a catalyst.
During the separation of erythrocytes from the fluid fraction it is further important that the red blood cells not lyse or rupture, which may cause the release of internal constituents into the plasma. In order to obtain plasma from whole blood for testing, basically four different kinds of techniques are used, viz. gravity, pressure drop, capillary flow and centrifugal force based techniques.
Assays which are to be performed in field conditions should be inexpensive, and the samples should be disposable. These aims cannot be properly met with some of the techniques mentioned above: pressure drop and centrifugal force require specialized equipment, such pumps and centrifuges making then unsuitable for field use, and gravity based techniques are too slow.
Therefore, assays for separating whole blood into red blood cells and plasma have been developed which are based on capillary flow in lateral flow assay cards. A number of such assays are disclosed in the art.
U.S. Pat. Nos. 4,477,575 and 4,816,224 describe the use of layers of glass microfibers to separate erythrocytes from whole blood. Papers containing 100% glass microfibers of the proposed kind are inherently weak and require extreme care in handling. Although strength can be enhanced through the use of binders, some binders can cause interference with the assay or make the sheet hydrophobic.
Another approach comprises the use of a single-layered medium made of a composite of glass microfibers, cellulose fibers and synthetic staple fibers. In this respect, reference is made to U.S. Pat. No. 5,186,843.
Further filter media are disclosed in US 2012/152859 and US 2003/175153.
Although the blood separation media proposed in the past are suitable for their intended purpose, some improvements are still desired.
Generally, filtration on lateral flow assay cards is hampered by inadequate speed, as whole blood is rather viscous and begins to clot in air. A standard blood filter must have pores smaller than the size of a red blood cell. This is due to the folding that the red blood cells are capable of. When these folded cells pass through the pores some of them become lodged in the pore, blocking it and decreasing the filtration efficiency. This also reduces the number of pores available for the passage of plasma and decreases the total amount of plasma that passes through the filter per unit time.
Further, damage or rupture of the cells may occur. This is especially true if the fiber that makes up the nonwoven is glass, such as the ones suggested above in U.S. Pat. No. 5,186,843.
Glass fibers still give good separation of erythrocytes from blood due to the affinity of red blood cells to glass which provides for efficient slowing up of the erythrocytes in capillary flow.